Editing Fahrenheit (section)

== Definition and conversion ==
{{temperature}}

Historically, on the Fahrenheit scale the [[freezing point]] of water was 32&nbsp;°F, and the [[boiling point]] was 212&nbsp;°F (at [[standard atmospheric pressure]]). This put the boiling and freezing points of water 180&nbsp;degrees apart.<ref name="rank" /> Therefore, a degree on the Fahrenheit scale was {{frac|1|180}} of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water were originally defined to be 100&nbsp;degrees apart. A temperature interval of 1&nbsp;°F was equal to an interval of {{frac|5|9}}&nbsp;degrees Celsius. With the Fahrenheit and Celsius scales now both defined by the [[kelvin]], this relationship was preserved, a temperature interval of 1&nbsp;°F being equal to an interval of {{frac|5|9}}&nbsp;K and of {{frac|5|9}}&nbsp;°C. The Fahrenheit and Celsius scales intersect numerically at −40 in the respective unit (i.e., −40&nbsp;°F corresponds to −40&nbsp;°C).

[[Absolute zero]] is 0 K, −273.15&nbsp;°C, or −459.67&nbsp;°F. The [[Rankine Scale|Rankine temperature scale]] uses degree intervals of the same size as those of the Fahrenheit scale, except that absolute zero is 0&nbsp;°R{{snd}} the same way that the [[Kelvin]] temperature scale matches the Celsius scale, except that absolute zero is 0&nbsp;K.<ref name="rank">{{cite book |pages = 273–274 |first = Walt |last = Boyes |url=https://books.google.com/books?id=ZvscLzOlkNgC&q=rankine%20temperature%20scale%20book&pg=PA273 |title = Instrumentation Reference Book |publisher = Butterworth-Heinemann |year = 2009 |access-date = 17 July 2011 |isbn = 978-0-7506-8308-1 }}</ref>

The combination of [[degree symbol]] (°) followed by an uppercase letter F is the conventional symbol for the Fahrenheit temperature scale. A number followed by this symbol (and separated from it with a space) denotes a specific temperature point (e.g., "[[Gallium]] melts at 85.5763&nbsp;°F"). A difference between temperatures or an uncertainty in temperature is also conventionally written the same way as well, e.g., "The output of the heat exchanger experiences an increase of 72&nbsp;°F" or "Our standard uncertainty is ±5&nbsp;°F". However, some authors instead use the notation "An increase of {{not a typo|50&nbsp;F°}}" (reversing the symbol order) to indicate temperature differences. Similar conventions exist for the Celsius scale, see {{section link|Celsius#Temperatures_and_intervals}}.<ref>{{cite book |last1=Dingman |first1=S. L. |title=Fluvial Hydraulics |date=26 February 2009 |publisher=Oxford University Press, USA |isbn=978-0-19-517286-7 |page=522 |url=https://books.google.com/books?id=cvwRDAAAQBAJ&pg=PA522 |language=en}}</ref><ref>{{cite book |last1=Sogin |first1=Harold H. |last2=Hassan |first2=Kamal-Eldin |title=A design manual for regenerative heat exchangers of the rotary type |date=June 1956 |publisher=Wright Air Development Center |page=xii |url=https://books.google.com/books?id=6cYnAAAAMAAJ&pg=RA9-PR12 |language=en}}</ref>

=== Conversion (specific temperature point) ===
For an exact conversion between degrees Fahrenheit and Celsius, and kelvins of ''a specific temperature point'', the following formulas can be applied. Here, {{math|''f''}} is the value in degrees Fahrenheit, {{math|''c''}} the value in degrees Celsius, and {{math|''k''}} the value in kelvins:
* {{math|''f''}}&nbsp;°F to {{math|''c''}}&nbsp;°C: {{math|''c''}} = {{sfrac|{{math|''f''}} − 32|1.8}}
* {{math|''c''}}&nbsp;°C to {{math|''f''}}&nbsp;°F: {{math|''f''}} = {{math|''c''}} × 1.8 + 32
* {{math|''f''}}&nbsp;°F to {{math|''k''}}&nbsp;K: {{math|''k''}} = {{sfrac|{{math|''f''}} + 459.67|1.8}}
* {{math|''k''}}&nbsp;K to {{math|''f''}}&nbsp;°F: {{math|''f''}} = {{math|''k''}} × 1.8 − 459.67

There is also an exact conversion between Celsius and Fahrenheit scales making use of the correspondence −40&nbsp;°F ≘ −40&nbsp;°C. Again, {{math|''f''}} is the numeric value in degrees Fahrenheit, and {{math|''c''}} the numeric value in degrees Celsius:
* {{math|''f''}}&nbsp;°F to {{math|''c''}}&nbsp;°C: {{math|''c''}} = {{sfrac|{{math|''f''}} + 40|1.8}} − 40
* {{math|''c''}}&nbsp;°C to {{math|''f''}}&nbsp;°F: {{math|''f''}} = ({{math|''c''}} + 40) × 1.8 − 40

=== Conversion (temperature difference or interval) ===
When converting a ''temperature interval'' between the Fahrenheit and Celsius scales, only the ratio is used, without any constant (in this case, the interval has the same numeric value in kelvins as in degrees Celsius):
* {{math|''f''}}&nbsp;°F to {{math|''c''}}&nbsp;°C or {{math|''k''}}&nbsp;K: {{math|''c''}} = {{math|''k''}} = {{sfrac|{{math|''f''}}|1.8}}
* {{math|''c''}}&nbsp;°C or {{math|''k''}}&nbsp;K to {{math|''f''}}&nbsp;°F: {{math|''f''}} = {{math|''c''}} × 1.8 = {{math|''k''}} × 1.8